feat: preview external audio errors

Co-authored-by: OpenCode (gpt-5.6-sol) <noreply@opencode.ai>

Author: hi-ogawa

packages/youtube-external-audio/src/web.tsx

@@ -36,6 +36,19 @@ function Web() {
     <main className="flex min-h-screen items-start justify-center bg-button p-8 font-sans text-foreground">
       <div className="flex flex-col items-end gap-3">
         <div className="flex gap-2">
+          <button
+            className="cursor-pointer rounded-md border border-button-border bg-panel px-2.5 py-1.5 text-xs hover:bg-button-hover"
+            type="button"
+            onClick={() =>
+              setError((current) =>
+                current
+                  ? undefined
+                  : "Audio is available for this session but could not be saved.",
+              )
+            }
+          >
+            {error ? "Clear error" : "Error preview"}
+          </button>
           <button
             className="cursor-pointer rounded-md border border-button-border bg-panel px-2.5 py-1.5 text-xs hover:bg-button-hover"
             type="button"